The present invention relates generally to the preparation of ferroelectric powders and thin films and more particularly to the preparation of solutionderived (Pb,La) (Nb,Sn,Zr,Ti)O.sub.3 thin films and powders.
Due to their electrical and electromechanical properties, thin films of ferroelectric materials are of great interest for a wide range of applications from nonvolatile random access memories (NVRAMs) to micro-electrical mechanical devices, to integratable capacitors. Ferroelectric materials are those that possess a spontaneous polarization that can be reoriented with an applied electric field. This leads to hysteresis in the polarization-voltage response and is the basis for NVRAM applications. The most widely studied class of ferroelectric thin films is based on the perovskite crystal structure of the lead zirconate titanate Pb(Zr.sub.1-x,Ti.sub.x)O.sub.3 ; PZT! family. Aliovalent cations (aliovalent indicates a dopant that has a different valence than the ion it was replacing, either higher or lower) are typically substituted for the A-site (Pb) and B-site (Zr,Ti) cations to modify the solid-state point defect chemistry and, thus, the electrical properties of the material. Lanthanum, an A-site donor, (La.sup.+3 substitutes for Pb.sup.+2) is used to reduce the coercive (switching voltage) and improve the dielectric constant, and reduced fatigue. Niobium and tin are B-site donor dopants (for example, Nb.sup.5+ substitutes for Ti.sup.4+ or Zr.sup.4+) and are typically used to reduce the coercive (switching) voltage and improve the insulation resistance of PZT thin films (see Tuttle, B. A.; Al-Shareef, H. N.; Warren, W. L.; Raymond, M. V.; Headley, T. J.; Voigt, J. A. Microelectronic Engineering, 1995, 29, 223).
Solution routes are widely used for the production of thin films through spin-cast or dip-coating methodologies. These methods are typically used due to the flexibility in the stoichiometry of precursor solutions, the ease of altering processing variables, cost effectiveness (inexpensive), the reduction of the sintering temperatures, and the integratability with existing semi-conductor processes.
The solution methodologies for the preparation of undoped and doped PZT precursor solutions can be divided into three general categories: metallo-organic decomposition (MOD), solution gelation (sol-gel) or hybrids. Both metallo-organic (MOD) routes which utilize large "soap-like" derivatives and the so-called "sol-gel" routes have been extensively studied.
Vest and Zhu (Ferroelectrics, 1991, 119, 61) describe an MOD process for preparing PZT materials by dissolving lead, zirconium, and titanium precursor compounds in xylene to produce a precursor solution which was then decomposed to subsequently produce the ferroelectric material.
Hampden-Smith, in U.S. Pat. No. 5,308,601 issued on May 3, 1994, also describes an MOD method for making metal oxides at low temperatures. Because of the problems associated with achieving adequate solubility, the method of Hampden-Smith requires synthesis of the starting materials as well as acid modification of all precursors and the addition of water.
The "sol-gel" routes which use 2-methoxyethanol or other "chelating" reagents i.e., the Inverted Mixing Order (IMO) method! typically involve altering the commercially available starting materials through the use of alcohol or acid solvents at elevated temperatures. Sol-gel processing generally suffers from limitations due to the relative solubilities of the various metals salts and metal alkoxides. Furthermore, controlling the stoichiometric ratios of two or more metals or metal oxides using sol-gel processing is complicated by the differing hydrolysis or precipitation rates of the precursors. Therefore, the starting materials are often modified to permit adequate solubility, adding processing steps to the method. These methods generally require synthesis of novel starting materials, relatively long mixing times, and/or heating during preparation of the desired precursor solutions. Furthermore, incorporation of lanthanum cations into the starting solutions is not easily accomplished, especially at higher La contents.
Ramamurthi and Payne (J. Am. Ceram. Soc., 1990, 73, 2547) describe a sol-gel method for making PT and PZT materials using alkoxides and acetates prepared in a methoxyethanol solvent system. The processing requires several hours as well as multiple processing steps to get the metal compounds in solution. These steps include refluxing at elevated temperature and hydrolysis.
Ravindranathan et al. (Ferroelectric Letters, 1990, 12, 29) describe a sol-gel process similar to that of Ramamurthi and Payne for making lead magnesium niobate. They also use methoxyethanol as the solvent system and require heating for several hours as well as hydrolysis of the precursor solution.
Miller et al., in U.S. Pat. No. 5,116,643, issued on May 26, 1992, as well as in U.S. Pat. No. 4,946,710 issued on Aug. 7, 1990, and in U.S. Pat. No. 5,028,455 issued on Jul. 2, 1991, describe a sol-gel method for producing ferroelectric thin films using alcohol and acid solvents in proportions sufficient to ensure equal reaction rates. However, Miller et al. also require heating to drive off the solvent, the addition of further reagents to quench reactivity and the introduction of water to hydrolyze the produced precursors.
Improvements to the preparation of PZT-family materials by these methods would be to reduce the preparation or synthesis time, to prepare the ferroelectric materials using commercially-available starting materials, and to reduce the severity of the preparation conditions, particularly the severity of the heating conditions. Once the precursor solutions have been prepared, either thin films or powders can be prepared by standard methods.